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Detection of Osmium

Metallic osmium is characterised by its volatility at white heat without liquefaction, and by the production of volatile and excessively poisonous osmium tetroxide upon ignition of the finely divided metal in air or oxygen. The same vapour is evolved when any osmium compound is heated with concentrated nitric acid.

Fuming nitric acid and aqua regia each dissolve the metal unless the last named has previously been very strongly heated, in which case it is insoluble in all acids.

When heated with potassium chloride in a current of chlorine, osmium yields the tetrachloride, OsCl4.

Reactions of Salts of Osmium

All compounds of osmium are reduced to the metal upon ignition in hydrogen. Hydrogen sulphide in acid solution effects the precipitation of brownish black osmium monosulphide, OsS, which is insoluble in ammonium sulphide.

Alkalies precipitate hydrated osmium dioxide, Os(OH)4, as a brownish red mass, whilst grey anhydrous osmium dioxide is obtained on fusing potassium hexachlor osmate, K3OsCl6, with sodium carbonate.

Osmium tetroxide dissolves in water, yielding a neutral solution possessed of oxidising activity. For example, it oxidises ferrous sulphate to the ferric condition, being itself reduced to the dioxide and thrown out of solution in the black hydrated condition, Os(OH)4 or OsO2. 2H2O.

Sulphurous acid or sulphur dioxide added to the solution yields a series of colour-changes ranging from yellow through green to blue, the last named being the colour of osmium sulphite, which then separates out. Metallic zinc effects the reduction and precipitation of metallic osmium from solution as a black powder, readily distinguished and separated from the allied platinum metals in that it is soluble in hydrogen peroxide.

A delicate colour-reaction for osmium consists in warming a solution containing the metal, in the form of its tetroxide or as chlorosmate, with thiocarbamide in excess, together with a few drops of hydrochloric acid. The presence of osmium is indicated by the development of a deep red or rose colour, due to the formation of a complex of composition. The colour is sufficiently intense to enable the detection of one part of osmium in 100,000.

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